A tandem vacuum booster for a brake system is disclosed in the Japanese Utility Model No. 61(1986)-44917. The tandem vacuum booster disclosed in this document includes a front shell and a rear shell which are connected to each other in order to form a housing. The tandem vacuum booster further includes a front diaphragm, an outer circumferential portion of which is fit between the front shell and the rear shell to fluidly separate a front variable pressure chamber from a front constant pressure chamber. Also included is a rear diaphragm having an outer circumferential portion that is fixed to a stepped portion of the rear shell in order to fluidly separate a rear variable pressure chamber from a rear constant pressure chamber. The tandem vacuum booster has a fixed wall formed by two plate members that are connected to each other by welding. One of the plates is fit between the front diaphragm and the rear shell at its outer circumferential portion and the other plate is fixed to the rear diaphragm to form the front variable pressure chamber and the rear constant pressure chamber.
Unfortunately, the tandem vacuum booster referenced above is quite costly to manufacture since the fixed wall is formed by two plate members. Further, because the plate members are welded to each other, the process of manufacturing the tandem vacuum booster is relatively complicated and this leads to additional manufacturing expense.
A vacuum booster for a brake system which is intended to address the above problems is disclosed in U.S. Pat. No. 5,014,597. This tandem vacuum booster includes a fixed wall having a bent portion which presses an outer circumferential portion of the front diaphragm to a vertical portion of the front shell in order to be fixed to the front shell and the rear shell. Additionally, an outer circumferential end portion of the fixed wall is fixed to the rear diaphragm. The rear shell of the tandem vacuum booster for a brake system includes a plurality of convex portions at an outer circumferential surface thereof which define air passages between the rear diaphragm and the rear shell. Further, the front diaphragm includes a plurality of slots forming passages in order to fluidly connect the front variable pressure chamber to the rear variable pressure chamber.
Since this tandem vacuum booster possesses a fixed wall formed by an integral plate, the tandem vacuum booster is relatively low in cost as compared to the other tandem vacuum booster described above. However, this tandem vacuum booster suffers from the following problems.
First, the dies necessary for forming the rear shell are rather complicated in construction. Further, a thin portion may be generated in the outer circumferential surface of the rear shell because of the convex portions of the rear shell.
Although it may be possible to provide a plurality of slits at the outer circumferential end of the rear diaphragm instead of the convex portions of the rear shell in an attempt to address this problem, an air passage cannot then be provided in this type of construction. Accordingly, the rear variable pressure chamber cannot be fluidly connected to the front variable pressure chamber.